1. Field of the Invention
The present invention relates to an inverter for a light source device (hereinafter, “light source inverter”) that drives a cold cathode fluorescent lamp (CCFL) serving as a light source, and more particularly to a light source inverter that drives a light source in which two CCFLs are arranged substantially in the shape of the letter U. The present invention also relates to a light source device including such a light source inverter and CCFL, and to a display device and a liquid crystal display device including such a light source device.
2. Description of the Related Art
Since display devices such as television sets and monitors are required to be compact today, and even small-sized electrical devices such as mobile telephones and PDAs (personal digital assistants) are provided with display devices, thin display devices such as liquid crystal display devices are desired. Liquid crystal display devices are provided with a liquid crystal panel containing liquid crystal in which the orientation of its molecules can be changed by the application of a voltage. Light is modulated by employing variations in optical characteristics such as optical rotary power corresponding to the change in the molecular orientation of the liquid crystal in the liquid crystal panel. Thus, light corresponding in intensity to the brightness of each pixel is transmitted to allow a display operation; the liquid crystal panel in itself, however, emits no light. For this reason, a light source is required that illuminates the liquid crystal panel. Hence, two types of liquid crystal display devices are available: transmissive liquid crystal display devices, where a backlight is employed as a light source; and reflective liquid crystal display devices, where an outside light source is employed.
Since transmissive liquid crystal display devices have higher color saturation than reflective liquid crystal display devices, and thus provide easy-to-view images even in poorly-lit indoor conditions, they are becoming increasingly used. Disadvantageously, however, transmissive liquid crystal display devices consume a large amount of electric power, and their displayed images are not sufficiently bright in well-lit outdoor conditions. For this reason, semi-transmissive liquid crystal display devices become available today in which a backlight is employed in a poorly-lit area, whereas outside light is employed in a well-lit area. There are two types of transmissive liquid crystal display devices: direct-type transmissive liquid crystal display devices incorporating a backlight in which a plurality of CCFLs are disposed immediately behind a liquid crystal panel; and edge-light type transmissive liquid crystal display devices incorporating a backlight that passes the light emitted from CCFLs disposed at the edge of the device via the flat surface of a light guide plate.
In transmissive liquid crystal display devices incorporating such backlights, a substantially U-shaped lamp composed of two CCFLs arranged side by side is used as a light source for the backlights. The CCFLs are driven so that the substantially U-shaped lamp emits light. Since an alternating-current power supply is required for driving the CCFLs, the transmissive liquid crystal display device incorporating the backlight is provided with a light source inverter for generating an alternating-current voltage.
As an example of the configuration of a light source inverter, a light source device that includes two substantially U-shaped lamps, each having two CCFLs is shown in FIG. 10. The light source inverter shown in FIG. 10 includes: transformers 102a and 102b stepping up alternating-current voltages to apply these stepped-up voltages to CCFLs 101a and 101b, respectively, in a substantially U-shaped lamp 101x; transformers 102c and 102d stepping up alternating-current voltages to apply these stepped-up voltages to CCFLs 101c and 101d, respectively, in a substantially U-shaped lamp 101y; rectifier circuits 103a and 103b connected to the secondary sides of the transformers 102a and 102b, respectively, to perform half-wave rectification; a stabilization circuit 104 receiving the currents that are half-wave rectified by the rectifier circuits 103a and 103b; a switching circuit 105a connected to the primary sides of the transformers 102a and 102b to perform power control on the primary sides of the transformers 102a and 102b; a switching circuit 105b connected to the primary sides of the transformers 102c and 102d to perform power control on the primary sides of the transformers 102c and 102d; and a control circuit 106 that sets switching frequencies for the switching circuits 105a and 105b according to an output from the stabilization circuit 104.
In the light source inverter of the configuration shown in FIG. 10, the currents received from the rectifier circuits 103a and 103b connected to the low-voltage sides of the transformers 102a and 102b connected to the substantially U-shaped lamp 101x are smoothed and compared with the reference value by the stabilization circuit 104. Based on this comparison result, the control circuit 106 operates the switching circuits 105a and 105b so as to stabilize the voltages outputted from the secondary sides of the transformers 102a to 102d connected to the substantially U-shaped lamps 101x and 101y. In other words, the switching circuits 105a and 105b each perform feedback operations based on the currents that are fed from the transformers 102a and 102b and are then monitored by the stabilization circuit 104.
In the configuration shown in FIG. 10, however, when the CCFLs 101a to 101d in the substantially U-shaped lamps 101x and 101y vary in impedance, or when their impedances vary due to uneven thermal distribution within the backlight, there is a possibility that a required voltage is not supplied to the substantially U-shaped lamp 101y. Thus, variations in the impedance of the CCFLs 101a to 101d in the substantially U-shaped lamps 101x and 101y may cause variations in the brightness of the substantially U-shaped lamps 101x and 101y. 
In particular, when a direct-type backlight is used in a liquid crystal display device having a large screen, a large number of substantially U-shaped lamps are arranged as shown in FIG. 11A. In this case, for example, when n substantially U-shaped lamps 101 are arranged, and the temperatures of the substantially U-shaped lamps 101 within a backlight decrease from the first row to the nth row as shown in FIG. 11B, the impedance of the substantially U-shaped lamp 101 in the first row more greatly differs from that of the substantially U-shaped lamp 101 in the nth row. Thus, for example, even if the rectifier circuits and the stabilization circuit are connected to the CCFLs 101a to 101b in the substantially U-shaped lamp 101 in the first row, and each of the voltages applied to the substantially U-shaped lamps 101 in the first to nth rows is attempted to be stabilized based on the current flowing through the substantially U-shaped lamp 101 in the first row, the currents flowing through the substantially U-shaped lamps 101 in the first to nth rows actually differ from each other.
In contrast, a backlight assembly is proposed in which a plurality of substantially U-shaped lamps are arranged, a stabilization circuit is connected to each of the substantially U-shaped lamps and feedback control is individually performed for each of the substantially U-shaped lamps (see patent document 1). When the configuration of the backlight assembly disclosed in patent document 1 is applied to that shown in FIG. 10, in the same manner that the rectifier circuits 103a and 103b and the stabilization circuit 104 are provided, two rectifier circuits connected to transformers 102c and 102d and the stabilization circuit that receives the half-wave rectified currents from the rectifier circuits are provided and a control circuit that controls the switching circuit 105b according to the output from the stabilization circuit is provided separately from the control circuit 106.
Like the backlight assembly in patent document 1, substantially U-shaped lamps, transformers and stabilization circuits are connected based on the connection relationship shown in FIG. 10. In this way, it is possible to eliminate differences between the currents generated when CCFLs are connected in parallel to one transformer and the complexity of interconnection caused by connecting a stabilization circuit to the CCFLs connected via a transformer in series. In the backlight assembly in patent document 1, a stabilization circuit is provided for each of substantially U-shaped lamps. Thus, unlike the case where the stabilization circuit is provided for only one substantially U-shaped lamp as shown in FIG. 10, since the backlight assembly in patent document 1 is provided with the stabilization circuit for each of the substantially U-shaped lamps, it is possible to control substantially U-shaped lamps individually, and also to eliminate differences between the currents flowing through the substantially U-shaped lamps.
Patent document 1: JP-A-2002-231034